1. Field of the Invention
The present invention relates to a silk hat shaped wave gear device in which a silk hat shaped flexible external gear is incorporated. More particularly, it relates to a silk hat shaped wave gear device in which the shape of a flexible external gear is improved in shape so that a wave generator can easily be incorporated into the flexible external gear.
2. Description of the Related Art
A wave gear device in which a flexible external gear has a silk hat shape as shown in FIGS. 1 and 2 is publicly known. In this specification, a wave gear device of this type is referred to as a silk hat shaped wave gear device.
As shown in FIGS. 1 and 2, a silk hat shaped wave gear device 11 includes a circular rigid internal gear 12, a silk hat shaped flexible external gear 13 disposed inside the rigid internal gear 12, and an elliptical wave generator 14 fitted into the flexible external gear 13.
The flexible external gear 13 has an annular body 22, a circular diaphragm 23 provided with an inner circumferential end portion 23a connected to a proximal opening of the body 22, and a thick circular boss 25 formed integrally on an outer circumferential end portion 23b of the diaphragm 23 so as to be connected thereto. External teeth 24 are formed integrally on an outer circumferential portion of the distal opening of the body 22 in a circumferential direction. The circular boss 25 is used to install the flexible external gear 13 to another member (not shown), and the body 22 and the diaphragm 23 are supported in a cantilever state by the boss 25.
The wave generator 14 includes a hollow hub 14a, an elliptical rigid cam plate 14b fitted on an outer circumference of the hollow hub 14a, and a ball bearing 14c fitted on an outer circumference of the rigid cam plate 14b. A portion of the body 22 where the external teeth 24 of the flexible external gear 13 are formed is deflected into an elliptical shape by the wave generator 14, whereby two external tooth portions located at both ends of a major axis of an ellipse are engaged with internal teeth 12a of the rigid internal gear 12. If the wave generator 14 rotates around a device axis 11a in this state, these engagement positions move in a circumferential direction. By this movement, a relative rotation is produced between the flexible external gear 13 and the rigid internal gear 12 according to a difference in the number of teeth between the external and internal gears. Therefore, for example, if the rigid internal gear 12 is fixed and the wave generator 14 is used as a high-speed rotation input element, the external gear 13 serves as a reduced-speed rotation output element, and a reduced-speed rotation output is obtained from the external gear 13.
In the silk hat shaped wave gear device 11 constructed as above, work of incorporating the wave generator 14 into the flexible external gear 13 is performed by inserting the wave generator 14 from an opening at a side where the external teeth 24 of the flexible external gear 13 are formed. However, it is necessary to insert the wave generator 14 having an elliptical contour into an external tooth forming portion of the flexible external gear 13 having a cylindrical shape, so that the assembling work is difficult to do. Although the body 22 has flexibility, the external tooth forming portion thereof is the most rigid portion in the body 22. Therefore, work of incorporating the wave generator 14 while the external tooth forming portion is deflected into an elliptical shape is troublesome and difficult.